To create a comet, just duplicate one of the entries in comets.sc and change the name and parameters.

ParentBody: What object the comet is orbiting. CometType: Periodic "P" or nonperiodic "C" Radius: in km. Not needed if using magnitude / slope parameter instead. Mass: in kg, divided by mass of Earth (6x1024kg). Not needed if using magnitude / slope parameter instead. RotationPeriod: Time to complete one rotation, in hours I believe. AbsMagn: Comet's intrinsic brightness, more negative is brighter. SlopeParam: Describes how brightness changes as a function of distance from sun.

Orbital Parameters. See wikipedia page if you have further questions. Epoch: Julian date at which the orbital elements are specified. Period: Time to complete one orbit in years. Very large for nonperiodic comets, and not used for hyperbolic comes (e.g. ISON) Eccentricity:Non-circularity of orbit. 0 for circle, 1 for parabola. Inclination: Tilt of orbit with respect to reference plane, in degrees. AscendingNode: Defines point where orbit crosses reference plane in northward direction, in degrees. ArgOfPericenter: Defines point of periapsis, in degrees. MeanAnomaly: Position of comet with respect to periapsis at the specified time, in degrees.

That is why all of my sungrazer comets disappeared so quickly, because I defined SlopeParam as 4.

Actually it's because SE doesn't properly calculate tail length, so sungrazers will only have long tails at perihelion, and be invisible at other times. I doubt there are any SlopeParam values you could use to get a realistic appearance for sungrazers.